This invention relates to glass compositions having improved total solar transmission, and improved refining and/or melting characteristics. In a conventional float line process, glass batch materials are heated in a furnace or melter to form a glass melt. The glass melt is poured onto a bath of molten tin (tin bath), where the glass melt is formed and continuously cooled to form a float glass ribbon. The float glass ribbon is cooled and cut to form solid glass articles, such as flat glass sheets. For float glass, the glass batch often includes soda, lime and silica to form soda-lime-silica based flat glass.
There is a tradeoff between glass production and the cost of manufacture. In particular, it is desirable to increase the rate of glass production but at the same time it is also desirable to reduce production costs. Certain glass manufacturers are operating their glass furnaces at higher and higher throughput and temperatures to meet the increased demand for glass. However, as more glass batch is processed, more fuel is required to melt the increased amounts of glass batch thereby increasing production costs and decreasing thermal efficiency.
Certain prior art has attempted to solve these problems. For example, U.S. Pat. No. 6,797,658 (the disclosure of which is hereby incorporated herein by reference) discloses decreasing the amount of MgO in the glass composition and increasing the amount of two or more of CaO, R2O (Na2O and K2O), Al2O3, and SiO2 by the same amount. The '658 patent contends that the melting and/or forming temperature of the glass can be reduced in such a manner. See also U.S. Pat. Nos. 6,878,652 (decreasing MgO and increasing CaO by the same amount), and 5,071,796, the disclosures of which are hereby incorporated herein by reference. However, these compositions are problematic for numerous reasons and do not provide for the best results.
Moreover, low iron glasses are known in the art. However, when low iron glasses are provided, there exists a need in the art to improve their transmission, including both their visible transmission and their total solar transmission. Total solar transmission is discussed herein in the context of ISO 9050, AM 1.5, which is incoporated herein by reference.
In view of the above, it will be apparent that there exists a need in the art for a glass having improved total solar transmission. There is also a need for a method of making a soda-lime-silica based glass composition which may realize a reduced refining time and/or increased refining rate. In certain example instances it would be desirable to provide a glass composition that is able to realize a lower viscosity so that refining of the melt occurs faster in the float line manufacturing process, and/or a method of making such glass.
Certain embodiments of this invention relate to a method of making soda-lime-silica based low iron (total iron no greater than about 0.04%) glass, and/or glass resulting therefrom. In certain example embodiments, boron oxide (e.g., such as boron trioxide, B2O3) has the unexpected and surprising effect in low-iron, high transmission, glass of improving total solar transmission. The boron oxide also is used for improving glass refining, homogeneity and quality (lower seed count) through its flux action and improves glass optical parameters of green and clear glass through the change in refractive index and surface tension. Boron oxide lends to broader and weaker absorption band of such transition element(s) as iron which additionally improves the transmittance of low iron clear glass in certain example embodiments of this invention. Glass according to certain example embodiments of this invention may be used in solar cell applications (where increased total solar transmission is desired), or in other suitable applications such as in the context of architectural windows or the like.
In certain example embodiments, the addition of boron oxide in certain quantities in advantageous in that it improves the chemical durability of the glass by decreasing the USPX (or USPX II or USPXIII) value of the glass via suppression Of the silica, sodium ions in the glass structure. In certain example embodiments, the USPX value is reduced to no greater than about 6.0, more preferably no greater than about 5.8, and most preferably no greater than about 5.75 (conventional glass made by the assignee of this application has a USPX value of about 6.2).
The boron oxide may be introduced into the glass batch or melt in the form of one or more of boric acid (H3BO3), sodium tetraborate decahydrate (Na2B4O7. 10H2O), sodium tetraborate pentahydrate, sodium pentahydrate (Na2B4O7.5H2O), or in any other suitable form. In certain example embodiments of this invention, the resulting soda-lime-silica based glass ends up including by weight percentage from about 0.1 to 3%, more preferably from about 0.1 to 2.5%, and most preferably from about 0.5 to 1.5% (e.g., about 1%), boron oxide (e.g., boron trioxide, B2O3). It has surprisingly been found that the use of boron oxide, and/or the form in which the same is introduced into the glass melt or batch, is advantageous in that it increases total solar transmission in low iron glasses, and permits the refining time of the glass to be substantially reduced (or the refining rate to be increased). Such glass compositions are useful, for example and without limitation; in solar cell applications, and in architectural, vehicular and/or residential glass window applications.
In certain example embodiments of this invention, there is provided a method of making soda-lime-silica based low iron glass comprising a base glass portion that includes: SiO2 67-75%, Na2O 10-20%, CaO 5-15%, Al2O3 0-7%, K2O 0-7%, the method comprising: providing boron oxide in a glass melt used in making the glass, the boron oxide acting to increase total solar transmission of the low iron glass, reduce refining time of the glass melt; and increasing a pull rate and/or reducing residence time of the glass melt in a refining zone of a glass manufacturing apparatus, compared to a situation where no boron oxide is present. In other example embodiments of this invention, there is provided a method of making soda-lime-silica based low iron glass, the method comprising: providing boron oxide in a glass melt used in making the soda-lime-silica based glass, in order to increase total solar transmission and reduce refining time of the glass melt.
In certain example embodiments, there is provided a glass comprising:
Ingredientwt. %SiO269-73%Na2O10-20%CaO5-15%MgO0-5%K2O0-2%total iron (expressed as Fe2O3)0.005 to 0.04%FeO0 to 0.0025%cerium oxide0 to 0.03%boron oxide0.1 to 2.5%SO30.1 to 0.6%wherein the glass has a glass redox of no greater than 0.12, a USPX value of no greater than 5.8, and a total solar transmission (ISO 9090 1.5 AM, at 3.2 mm reference thickness) of at least 90.8%. This 3.2 mm glass thickness is for purposes of reference only for purposes of total solar transmission measurement, and is not limiting as to how thick the glass may be according to the claimed invention.